vfs2: add fill_in_metadata_if_dir and ensure_item_has_metadata

vfs2: add fill_in_metadata_if_dir and ensure_item_has_metadata

Add fill_in_metadata_if_dir() as a contents() helper since contents()
does not return metadata for directories (because it's expensive).

This supports:

  (name, fill_in_metadata_if_dir(repo, item)
   for name, item in tuple(contents(...)))

Add ensure_item_has_metadata() to combine fill_in_metadata_if_dir()
and augment_item_meta() for cases like "ls -l" where you want
every item to have all the metadata available, and to have a Metadata
instance (even if faked), not just a mode.

Signed-off-by: Rob Browning <rlb@defaultvalue.org>
Tested-by: Rob Browning <rlb@defaultvalue.org>